The present invention pertains to a method of molding completely closed, thin-walled, seamless hollow articles of fusible, or thermoplastic resin, such as polyethylene for example, without providing a vent hole in the article to vent the air enclosed within the article. More specifically, the invention pertains to a method of molding hollow fishing buoys, and also the fishing buoys or other articles produced by the method.
Plastic fishing buoys used in arctic seas are exposed to extremely severe conditions, owing to the cold temperatures, severe tidal surges that cause the buoys to be submerged to considerable depths, attacks by sea lions and other marine creatures, impact against rocks and the sides of fishing boats, and rough handling, all of which contribute to wear and tear on the buoys. The first molded plastic buoys were made of two molded plastic halves that were welded together, but the weld seam proved to be a line of weakness that quickly failed under the severe stresses of arctic usage. One-piece, seamless buoys made by the rotational molding process were tried next, but the plug that was used to seal the vent hole which has always been, until now, necessary when molding hollow parts by the rotational molding system, has proved to be a serious weakness that fails under stress, with consequent loss of the buoys and the traps to which they were attached. Vent holes in rotationally molded, completely closed, hollow parts have heretofore been considered absolutely essential to the molding process, to allow air to move into or out of the hollow part as the air heats up and expands, or cools and contracts. If no vent hole is provided, the hollow part will partially collapse as it cools after being molded, owing to the partial vacuum produced by contraction of the air trapped inside. This can be overcome by considerably increasing the wall thickness of the part, based on the physical properties of the plastic material at the heat distortion temperature, but this results in an overdesigned part which takes more material, is more costly to manufacture and is heavier to use. For this reason, conventional rotational molding processes require a venting hole in the part to prevent formation of a partial vacuum inside the buoy. This vent hole is then sealed, which introduces the possibility of a leak at the seal.